1. Field of the Invention
The present invention relates to a surface emitting laser and, in particular, to a high-power surface emitting laser.
2. Description of the Related Art
Recent attention has focused on a surface emitting laser as a light source for optical communication, optical transmission or electrophotography.
The surface emitting laser has excellent characteristics such as a single longitudinal mode, a low threshold and easy two-dimensional array formation.
For such a surface emitting laser, development of two-dimensional photonic crystal surface emitting laser utilizing a photonic crystal has been accelerated.
The photonic crystal used herein is a structure having a refractive index period equal to or smaller than the wavelength of light.
A two-dimensional photonic crystal surface emitting laser as disclosed in Japanese Patent Application Laid-Open No. 2003-273453 has a photonic crystal periodic structure arranged with a refractive index period in a two-dimensional manner in the vicinity of an active layer which light-emits by injecting carriers and is adapted to perform surface emitting with resonance by a photonic crystal.
The surface emitting laser disclosed in Japanese Patent Application Laid-Open No. 2003-273453 described above, constituting a surface emitting laser with unnecessary regrowth or laminating process, specifically, has such a structure as illustrated in FIG. 5.
The surface emitting laser, as illustrated in FIG. 5, has a lower clad 12, an active layer 13 and an upper clad 14 layered on a substrate 11.
At the central portion of the upper clad layer 14, there is provided a light-emitting region having a photonic crystal periodic structure 21 as a laser resonator and, around thereof, a ring-shaped upper electrode 17 is disposed.
The surface emitting laser is adapted to cause the active layer 13 to emit light for laser oscillation by injecting carriers into the active layer from a lower electrode 16 under the upper electrode 17 and the substrate 11.
On the other hand, U.S. Pat. No. 6,154,480 has proposed a vertical resonator type surface emitting laser constructed as a resonator mirror by utilizing guided mode resonance, hereinafter referred to as “GMR”, using a slab having a photonic crystal. In the surface emitting laser, the photonic crystal formed on a slab guided wave path makes a guided wave mode of the slab guided wave path resonate with a radiation mode for emitting light to the outside of the slab. Thus, the photonic crystal slab can remarkably decrease the transmittance of the incident light from the direction perpendicular to a surface of the slab guided wave path or can remarkably increase reflectance in a resonance wavelength of GMR. Specifically, the slab guided wave path having a photonic crystal structure can have a function as a mirror. The mirror having a photonic crystal structure and another mirror (for example, another mirror of the same type or multi-layer film reflector) are faced each other, sandwiching an active layer to constitute a vertical resonator. The surface emitting laser, which performs laser oscillation by injecting carriers into the active layer from an electrode through the photonic crystal structure mirror, has been disclosed as one embodiment.
However, the surface emitting lasers of conventional examples disclosed in Japanese Patent Application Laid-Open No. 2003-273453 and U.S. Pat. No. 6,154,480 have following problem in attaining a high power.
In the surface emitting laser utilizing a photonic crystal layer, it is desirable that an electrode for injecting a current into the active layer does not absorb the light generated at the active layer as much as possible, from the viewpoint of light emitting efficiency of laser. Generally, the electrode is disposed in a location other than the vicinity of a photonic crystal region. This is because in Japanese Patent Application Laid-Open No. 2003-273453 and U.S. Pat. No. 6,154,480, light resonates to increase the light intensity in the photonic crystal region and, if an absorber exists in the vicinity thereof, resonant light is strongly absorbed there. Accordingly, Japanese Patent Application Laid-Open No. 2003-273453 has proposed that a ring-shaped electrode is arranged around so as to surround a photonic crystal, thereby restraining light absorption.
Under such a configuration, if the area of a photonic crystal region is widened to attain a high power, the ring inner diameter of a ring-shaped electrode surrounding the photonic crystal region must be also increased. As a result, there occurs a disadvantage that carrier injection is not performed fully enough to reach an active layer. Specifically, the diffusion length of carriers in a semiconductor is not so large, so that the carriers are not fully injected to reach the central portion of a light-emitting region positioned under the center of the ring-shaped electrode and formed from the active layer. Accordingly, current injection efficiency drops, which makes it difficult to obtain a high laser beam output.
On the other hand, by providing an electrode or a conductive film over the whole surface of the top of the photonic crystal, placing priority on efficient carrier injection into the active layer, light absorption is intensified by the electrode or conductive film. Thus, a high laser beam output is difficult to obtain.
It is an object of the present invention to provide a surface emitting laser capable of solving the foregoing problems and suppressing light absorption as well as attaining highly efficient current injection into an active layer to attain a high power.